Conventional tellurite glass includes the following two types, which are useful as high-refraction, high-dispersion optical glass and a medium for acoustic optics:
(1) Tellurite glass consisting of 50 to 65 mol% of TeO.sub.2, 20 to 30 mol% of WO.sub.3 and 10 to 20 mol% of Li.sub.2 O as basic glass formers; and one or more of 2 to 10 mol% of K.sub.2 O, 1 to 4 mol% of MgO, 1 to 6 mol% of BaO, 1 to 8 mol% of ZnO, 1 to 5 mol% of CdO, 1.5 to 6 mol% of TiO.sub.2, 0.5 to 10 mol% of PbO, 0.5 to 5 mol% of La.sub.2 O.sub.3, 1 to 6 mol% of B.sub.2 O.sub.3, 1 to 6 mol% of Nb.sub.2 O.sub.5 and 2 to 8 mol% of Bi.sub.2 O.sub.3 to make 100 mol%, as described in Japanese Patent Publication No. 9083/73, hereinafter referred to as "Conventional Glass 1".
(2) Tellurite glass consisting of 60 to 75 mol% of TeO.sub.2, 5 to 20 mol% of ZnO, 5 to 20 mol% in total of Na.sub.2 O and Li.sub.2 O, up to 15 mol% of PbO, up to 16 mol% of BaO and up to 10 mol% of La.sub.2 O.sub.3, as described in Japanese Patent Publication No. 28454/77, hereinafter referred to as "Conventional Glass 2".
Conventional Glass 1 is stable to devitrification and has improved chemical durability. Conventional Glass 2 has high figure of merit (Me value) and low ultrasonic absorbency.
In a typical application of tellurite glass, the acoustic optical modulation element is essentially composed of an acoustic optical medium made of tellurite glass in a block form, a transducer adhered on the upper side of the medium and a sound absorbing material placed beneath the lower side of the medium opposite to the transducer. In this element, a modulated signal is put into the transducer and converted to an ultrasonic signal. The ultrasonic signal is transmitted through the medium. On the other hand when a laser beam is entered from the side of the medium at a Bragg angle .theta..sub.B to the wave surface of the transmitting ultrasonic wave, the laser beam emerged from the medium includes not only 0-order ray that has transmitted in straight line but also first-order diffracted beam through an angle of 2.theta..sub.B to the light path of the 0-order beam.
However, when Conventional Glass 1 or 2 is applied to such an acoustic optical medium for acoustic optics, the aforesaid first-order diffracted laser beam undergoes positional drift. The degree of the positional drift is usually from 0.1 to 0.15 mrad with an electric power applied to the transducer being 1 W. Such a drift is believed to be caused by a temperature increase of the transducer, adhesive layer and medium. Such an increase in temperature brings about change in refractive index of the medium, and it is considered that the degree of a positional drift depends on changes in optical pass due to changes in temperature (ds/dt) as represented by the following formula: EQU ds/dt=(dn/dt)+.alpha.(n-1)
wherein dn/dt is a difference in refractive index (n) with a temperature difference; and .alpha. is an expansion coefficient.